db/Count: print empty group if song without grouped tag exists

Be consistent with "list" responses.

NEWS

@@ -1,6 +1,7 @@
 ver 0.20.22 (not yet released)
 * protocol
   - add tag fallbacks for AlbumArtistSort, ArtistSort
+  - "count group ..." can print an empty group
 * storage
   - curl: URL-encode paths
 * Android

doc/protocol.xml

@@ -1599,6 +1599,11 @@ OK
               per-artist counts:
             </para>
             <programlisting>count group artist</programlisting>
+            <para>
+              A group with an empty value contains counts of matching
+              song which don't this group tag.  It exists only if at
+              least one such song is found.
+            </para>
           </listitem>
         </varlistentry>
 

src/db/Count.cxx

@@ -90,7 +90,7 @@ GroupCountVisitor(TagCountMap &map, TagType group, const LightSong &song)
 	assert(song.tag != nullptr);
 
 	const Tag &tag = *song.tag;
-	VisitTagWithFallback(tag, group,
+	VisitTagWithFallbackOrEmpty(tag, group,
 				    std::bind(CollectGroupCounts, std::ref(map),
 					      std::cref(tag),
 					      std::placeholders::_1));

src/tag/VisitFallback.hxx

@@ -49,4 +49,12 @@ VisitTagWithFallback(const Tag &tag, TagType type, F &&f) noexcept
 				    });
 }
 
+template<typename F>
+void
+VisitTagWithFallbackOrEmpty(const Tag &tag, TagType type, F &&f) noexcept
+{
+	if (!VisitTagWithFallback(tag, type, f))
+		f("");
+}
+
 #endif